The widely accepted theory that changes in Earth's
orbit drive cycles of glaciation can't account for an early thawing of glaciers
from the next-to-last ice age, according to research at the University of Minnesota.
The failure of the Milankovitch theory, also called orbital forcing, to predict
this thawing points to the existence of other factors that can override orbital
forcing to influence climate, the researchers said. The work will be published
in the Jan. 11 issue of Science.

Relying on extensive examinations of fossil
corals, the researchers have tracked sea level changes and volumes of glacial
ice, said lead author Christina Gallup, an assistant professor of geology at the
University of Minnesota-Duluth. Because sea levels fall when glaciers build up
and rise during glacial melting, the elevation at which corals grow depends partly
on the glacial cycle. The Minnesota team has dated many ancient corals from Barbados
beaches and determined when they grew and at what elevation with respect to current
sea level. They have used their findings to compare glacial cycles to cycles in
Earth's orbit, which, according to the theory of Serbian scientist Milutin Milankovitch
(1879-1958), provide the impetus for ice buildup and melting.

"All
previous coral data are in accord with Milankovitch," said Gallup. "Scientists
have followed ice ages as far back as three million years and always found frequencies
in the marine record of glaciations that are the same as the frequencies of variation
in Earth's orbit.

"In this study, we did a more thorough sampling
of the coral terrace built during the interglacial [between ice ages] period that
occurred between 130,000 and 120,000 years ago. We found corals whose age and
position indicated that during the transition to that interglacial, sea level
rose too early to be consistent with Milankovitch. This exception tells us something.
It implies that other things can override orbital forcing of glacial cycles."

The
Milankovitch theory says that summer heating at high northern latitudes drives
the melting of glaciers, Gallup explained. At latitude 65 degrees north, changes
in Earth's orbit would have caused summer heating to reach a minimum at about
140,000 years ago and increase to a peak at 129,000 years ago. The theory predicts
that most of the melting would occur after the halfway point, that is, 134,000
years ago and later. But the corals from Barbados indicate that sea level was
quite high--within 20 percent of its all-time high--136,000 years ago. Therefore,
much melting must have occurred thousands of years before the halfway point.

The
early melting of glaciers could have been due to one or more factors, said Gallup.
Scientists have theorized that when ice sheets get heavy enough, they depress
the land beneath them and sink. They encounter warmer air at lower elevations,
they have less area that's accumulating snow and ice, and sometimes, seawater
flows under their bottom layers and helps melt them further. Also, the height
of an ice age probably experiences the biggest amounts of floating sea ice. This
hinders evaporation of water, shutting off the supply of moisture that fuels snowfalls
at high latitudes.

A third factor was suggested by Robert Johnson, a geologist
at the University of Minnesota-Twin Cities. Normally, as one moves from tropical
to polar latitudes, there is a strong gradient in the amount of solar radiation
received. But if this gradient and its accompanying temperature contrasts should
weaken, then there will be a lesser energy gradient to drive moisture northward.

"Bob found that there was a minimum in this gradient 140,000 years
ago," said Gallup. "This would contribute to glaciers deflating. They
also could have been starved by sea ice and destabilized by sinking."

Gallup's
colleagues in the study were University of Minnesota-Twin Cities geology professor
Lawrence Edwards and postdoctoral fellow Hai Cheng, along with Fred Taylor, a
faculty member at the University of Texas at Austin. The work was supported by
the National Science Foundation.